The present invention relates to a process for the preparation of water-absorbing polymer structures, to the water-absorbing polymer structures obtainable by that process, to water-absorbing polymer structures, to a composite comprising a water-absorbing polymer structure and a substrate, to a process for the preparation of a composite, to the composite obtainable by that process, to chemical products, such as foams, molded articles, and fibers comprising water-absorbing polymer structures or a composite, to the use of water-absorbing polymer structures or of a composite in chemical products, and to the use of a salt in the treatment of the surface of water-absorbing polymer structures.
Superabsorbers are water-insoluble, crosslinked polymers which are able to absorb large amounts of aqueous fluids, especially body fluids, such as urine or blood, with swelling and the formation of hydrogels, and to retain such fluids under a certain pressure. By virtue of those characteristic properties, such polymers may be used for incorporation into sanitary articles, such as, for example, baby's diapers, incontinence products, or sanitary towels.
The preparation of superabsorbers is generally carried out by free-radical polymerization of acid-group-carrying monomers in the presence of crosslinkers. It is possible for polymers, having different absorber properties, to be prepared by the choice of the monomer composition, the crosslinkers, and the polymerization conditions, and of the processing conditions for the hydrogel obtained after the polymerization. Further possibilities are offered by the preparation of graft polymerizates, for example using chemically modified starch, cellulose, and polyvinyl alcohol according to DE-OS 26 12 846.
Commercially available superabsorbers are essentially crosslinked polyacrylic acids, or crosslinked starch/acrylic acid graft polymerizates in which the carboxyl groups have been partially neutralized with sodium hydroxide solution or potassium hydroxide solution.
For aesthetic reasons and environmental considerations, there is an increasing tendency to make sanitary articles smaller and thinner. In order to maintain the total retention capacity of the sanitary articles, the requirements of smaller and thinner can be met only by reducing the proportion of high-volume fluff. As a result, the superabsorber has to fulfill further functions in respect of transport and distribution of fluid, which can be summarized as permeability properties.
Permeability, in the case of superabsorber materials, generally means the ability in the swollen state to transport added fluids and distribute them three-dimensionally. In the swollen superabsorber gel, that process takes place by means of capillary transport through interstices between the gel particles. Transport of fluid by swollen superabsorber particles themselves obeys the laws of diffusion and is a very slow process which, in the situation of being used in a sanitary article, plays no role in the distribution of the fluid. In the case of superabsorber materials unable to affect capillary transport on account of lack of gel stability, separation of the particles from one another was ensured by embedding those materials in a fiber matrix, the gel blocking phenomenon being avoided. In new generation nappy/diaper structures, only very little fiber material, or none at all, is present in the absorber layer to assist the transport of fluid. The superabsorbers used therein must accordingly have sufficiently high stability in the swollen state so that the swollen gel still has a sufficient amount of capillary spaces through which fluid can be transported.
In order to obtain superabsorber materials having high gel stability, one possibility is to increase the degree of crosslinking of the polymer, which inevitably results in a reduction in swelling ability and retention capacity. An optimized combination of different crosslinkers and co-monomers, as described in DE 196 46 484, is able to improve the permeability properties but not to a level which allows, for example, a layer possibly consisting only of superabsorbers to be incorporated into a nappy/diaper structure.
Furthermore, methods for the after treatment of the surface of polymer particles can be used to improve the superabsorber properties. Surface treatments known from the prior art include, for example, post-crosslinking of the absorbent polymer structure at the surface, bringing the surface into contact with inorganic compounds, and post-crosslinking of the surface in the presence of inorganic compounds.
For example, DE 199 09 653 A1 and DE 199 09 838 A1 disclose powdered polymerizates post-crosslinked at the surface which absorb water, aqueous, or serous fluids, or blood, which polymerizates are based on acid-group-carrying monomers and have been coated with a surface post-crosslinking agent and, for example, aluminum sulphate in aqueous solution and post-crosslinked. The polymerizates disclosed in that prior art have advantageous absorption properties in comparison with conventional polymerizates, especially a high degree of permeability.
DE 102 49 821 A1 describes likewise powdered, water-absorbing polymer structures that are post-crosslinked at the surface having a high degree of permeability, which polymer structures have been obtained by surface treatment of untreated, water-absorbing polymer structures with a mixture of a crosslinker and an inorganic sol, for example silicic acid sol.
The disadvantage of the surface modification measures known from the prior art, especially the treatment of water-absorbing polymer structures with inorganic salts or inorganic sols for the purposes of improving permeability, lies, however, particularly in the fact that the increase in permeability that is observed is frequently also associated with a significant reduction in absorption capacity under pressure. That reduction in absorption capacity under pressure often results in the hygiene articles' having an increased tendency to leak after being wetted once or, especially, several times by the user.
The problem underlying the present invention was to overcome the disadvantages arising from the prior art.
The problem underlying the present invention was especially to provide a superabsorber which firstly promotes the transport of aqueous fluids, such as urine, in hygiene articles such as absorbent layers or cores, particularly for diapers, having high superabsorber concentrations, and thus to increase the wearing comfort of such hygiene articles.
A further problem underlying the present invention was to provide a process by which such advantageous superabsorbers and a composite comprising such superabsorbers can be produced.
Another problem underlying the present invention was to provide hygiene articles that are as thin as possible and have a high content of water-absorbing polymer structures and that are characterized by good in-use properties.